Homebrewed Electricity > Wind
Please discuss stalling and adding resistance to the line.
WoodWaterWheel:
Chris, Your method of adding variable restiance using the springs is great. kudos
Rainwulf:
This looks like something that a reprogrammed MPPT controller could deal with.
Using PWM, and off cycle voltage measurements to measure the actual voltage (like they do with brushless motors, as this is basically a brushless motor in reverse), you could do whats called "active PFC".
The problem with AC alternators and supplies is that when rectified, you are only using the very peak of the ac waveform, as that's the only time when the diodes are forward biased with respect to the battery bank or dc rail or capacitor bank.
During the other times, there is zero load, zero current flow, so the rotor sees less "loading" and will spin up faster. It also means that instead of a nice regular current flow, the current flow ends up as a series of short pulses of very high current, with the rest of the time no current at all is flowing. Due to the fairly large current pulses, you need large cables to help reduce I2R losses.
What active PFC does is engage a boost regulator during the up and down sides of the waveform. The regulator tracks the sine wave, and engages higher boost when the waveform is closer to zero volts. What this does is allow you to pull power out of 70-80 percent of the waveform, instead of only the top 5 percent of the peak.
Once thats done, you end up with a fairly stable actively rectified DC rail. You then use standard MPPT controller schemes in conjunction with the aforementioned RPM and voltage detection to intelligently vary the load the wind generator sees. Basically, instead of a standard ohmic load like a battery bank or series of resistors, you load the generator down with an "active" load. This would be ideal during low wind and a flat battery bank, as you could reduce the boost voltage to reduce the load on the generator, getting it to spool out of stall speeds. Basically, MPPT.
As your battery bank charges and you end up with excess electricity you could simply shunt regulate the alternator using high frequency PWM.
You could approximate the first half of this system with a high power DC-DC buck regulator that runs of the rectified but not capacitively filtered output. The second half, the shunt regulator, would have to be switched in at full charge, but then act like a solar panel PWM shunt regulator.
Just my 2 cents. I could be very wrong as i dont have a wind generator.
SparWeb:
Hi Rainwulf,
There are actually two subjects to deal with here. You've put your finger on one of them, but is not always the chief factor. You can explore some discussions on this matter that I've had recently with another member "Dave P68" where we thrashed out some details, and then he ran a bunch of tests to figure it out.
The other factor, put simply, is when the blades just don't have the torque to drive the alternator. It's a balancing act: blades too big and the thing runs away in high wind, blades too small and it never starts except in high winds! What the resistors do, when put in series on the power lines, is to reduce the current in the alternator, which also reduces the torque it imposes on the shaft to the blades.
Rainwulf:
Yep i understand. What you could do is use a second input, that being windspeed using either a hot wire element or a small cup anemometer.
if your controller detects that there should be more output with respect to windspeed, digitally remove some of the electronic loading by reducing the boost voltage target. That would unload the alternator allowing it to spin up.
SparWeb:
Hi Rainwulf,
Maybe you'd find this (rather old) discussion interesting:
https://www.fieldlines.com/index.php/topic,127288.0.html
Flux built the discussion and details slowly, but very thoroughly examined buck-boost options for making the match.
When I read your post from Saturday I got a little sidetracked by the term "active PFC" but now I see you really wanted to talk about DC buck/boost.
I have not personally delved into the MPPT world, either as a user of off-the-shelf equipment or as a DIY builder.
Regular hard-wired connections through rectifiers don't suffer from too much inefficiency, and their chief advantage is being dead simple and reliable. In this scheme, the EMF generated by the alternator grows linearly with speed. In high winds this EMF can be many times the voltage at the battery. When rectified with a 3-phase rectifier, the whole electrical system is clamped to the battery voltage. Roughly speaking:
I = (EMF - Vbatt - Vf) / Z
In words, that says the current equals the difference between the generator electromotive force and the battery voltage minus the rectifier forward voltage drop, divided by the total system impedance. In practice, the equation needs adjustment to account for the AC EMF and the DC battery voltage - it's just to give you an idea.
There is a time near "cut-in" where the EMF is only barely higher than the battery voltage, and the "short pulses" do happen as you say, but the current is very low. Once the EMF rises, then there's plenty of delta-V over the battery voltage for a majority of the sine for current to flow most of the time, and this situation gets set up at RPM not much higher than cut-in. From there, the AC waveform stays pretty "square" as RPM increases. Different mills respond differently, and mine is a horrible mess, but in the Axial-flux machines they are neat and tidy. One major difference between wind turbines and solar panels is that solar energy always tops out at about 1000 W/m^2 and many solar panels start to produce a trickle at 100 W/m^2 or so - a factor of 10x. The power in the wind rises as the cube of the wind speed, so for a turbine that cuts-in at 10mph wind and furls at 40mph, the range of power it has to accept is nearly 100x.
The big advantage you get from MPPT or a DIY buck or boost controller is that you don't have to get the blades perfectly matched to the alternator, and in fact you can do other tricks that allow you to improve even a well-matched set of blades and alternator. That comes from the difference in power curve between passive electrical machines and wind power. They never do match; you can only get them close. With MPPT, you can make them much much closer, keeping the turbine running at peak blade efficiency at a wider range of wind speeds. Without it, you can only pick one wind speed to optimize, and accept less than optimal matching at other wind speeds. This one feature is what offers a 50% increase in energy yield from a MPPT-controlled turbine.
The electronic scheme that the MPPT controller uses is not, I don't think, like power factor correction (PFC). I believe units like the Outback use high-frequency switching - much higher than the turbine's generator frequency - to manage the load. The generator may be giving 60-100Hz but the controller is switching at many kHz, and sampling the turbine output frequency/voltage frequently to sense the turbine conditions to be regulated. I believe it's closer to your last suggestion "...you could simply shunt regulate the alternator using high frequency PWM." but I don't believe it's simple. In a wind turbine, there is no condition where a disconnect is permissible. Any scheme used for regulation has to fail-safe. This can be done with a shunt regulator, and with the controller shut down or failed the load remains connected to the turbine.
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